In a mobile communication system, it is common that a terminal (to be referred to as “user apparatus UE” hereinafter) and a base station eNB perform communication so that communication is performed between user apparatuses UE. However, in recent years, various techniques are considered for performing direct communication between user apparatuses UE (non-patent document 1, for example). When performing communication between user apparatuses UE, a user apparatus UE receives a discovery signal from another neighboring user apparatus UE, so that the user apparatus UE discovers the other user apparatus UE that becomes a communication partner so as to perform D2D communication with the user apparatus UE.
According to D2D communication, traffic between UE-eNB can be offloaded, and communication can be performed even when a base station eNB cannot perform communication due to disaster and the like.
On the other hand, in the LTE (Long Term Evolution) system, packet based radio communication is utilized. In such a packet based radio communication, a plurality of hierarchical layers execute various kinds of functions for radio communication, so that radio communication is realized. In LTE, it is defined that radio communication is realized by a layered structure of radio interface protocols shown in FIG. 1.
The radio interface protocols shown in FIG. 1 include a PHY (Physical) layer, a MAC (Media Access Control) layer, a RLC (Radio Link Control) layer, a PDCP (Packet Data Convergence Protocol) layer, and a RRC (Radio Resource Control) layer. Although the radio interface protocols are protocols between a user apparatus UE and a base station eNB, protocols similar to the radio interface protocols shown in FIG. 1 may be utilized in D2D communication between user apparatuses UE.
In the protocols shown in FIG. 1, the PDCP layer which is a sublayer of layer 2 provides with functions such as secrecy processing, tampering detection, and header compression and the like of an IP (Internet Protocol) packet. A COUNT value that includes HFN (Hyper Frame Number) and PDCP SN (Sequence Number) is used for secrecy processing and tampering detection.
PDCP SN is incremented each time when a PDCP packet is transmitted from the PDCP layer to the RLC layer, and for example, a PDCP SN within a range from “0” to “4095” is cyclically provided to a PDCP packet. For example, a sequence number “0” is provided to a PDCP packet that is transmitted to the RLC layer next to a PDCP packet of a sequence number “4095”. HFN is incremented each time when the PDCP SN goes round. The COUNT number is formed such that it has HFN in upper bits, and PDCP SN in lower bits. In the LTE system, only a PDCP SN of the COUNT value is transmitted to a receiving side. HFN is not transmitted. Therefore, the receiving side estimates HFN of a received packet based on reception status.
Outline of operation of the PDCP layer is as follows. In a transmission side, a PDCP entity executes secrecy processing, tamper detection and header compression on the packet received from an upper layer, that is, on a PDCP SDU (Service Data Unit), by using the COUNT value, and adds a PDCP SN to the header so as to transmit the packet as a PDCP packet, that is, as a PDCP PDU (Packet Data Unit) to the RLC layer.
On the other hand, in the receiving side, as shown in FIG. 2, the PDCP entity manages a reception window of a predetermined size for correcting the order of received packets. In a case where a PDCP SN of a packet received from the transmission side falls within a reception window, the PDCP entity estimates an HFN used for deciphering of the packet based on the current reception status, and executes deciphering on the received packet based on the COUNT number formed by the estimated HFN and a PDCP SN of the header. After that, the PDCP entity transmits the processed packet to an upper layer to update the reception window. On the other hand, in a case where a PDCP SN of the packet received from the transmission side is outside the range of the reception window, the PDCP entity discards the packet.
Next, outline of the RLC layer is described. In the RLC layer, window control is performed in the transmission side and the reception side for providing order control and duplication control.
Tx window is managed in the transmission side, and Rx window is managed in the reception side. The transmission side adds a sequence number (SN) each time when transmitting a new RLC PDU so as to transmit a RLC PDU including the SN to the reception side. In the transmission side, transmitted RLC PDU is stored in a buffer, and the RLC PDU is discarded after checking of Status report(ACK) from the reception side.
The Rx window in the reception side is updated when the reception side receives RLC PDUs in the order of SN without any drop. Also, Tx window of the transmission side is updated based on Status report from the reception side.
As exemplary shown in FIG. 3, in a case where the transmission side cannot receive Status report from the reception side, the transmission side cannot update Tx window. As a result, Tx window stalling occurs so that new data cannot be transmitted. At this time, the reception side cannot receive the new data. ProhibitTimer shown in FIG. 3 is a timer for reducing an overhead due to frequent transmission of Status report. ProhibitTimer starts at the time of transmission of Status report, and transmission is not performed even when Status report is triggered during the timer is running.
As described above, in RLC/PDCP, window control is performed based on the SN added in the transmission side, and, for example, when the reception side receives data of a SN outside of the window, the data is discarded.